The present invention relates to self-steering axle assemblies for road vehicles, such as may be used for primary and/or auxiliary axle assemblies of commercial vehicles as dictated by loading conditions and highway regulations.
Multiple axle assemblies are typically employed for load distribution on trucks and trailers for hauling heavy loads. In contrast with the simple case of only two axles, one of which is steerable, greater numbers of axles give rise to a need for self-steering to avoid “scrubbing” of tires, unless a positive proportional steering mechanism is utilized. The added cost and specialized nature of such mechanisms makes them prohibitive in many applications, particularly for auxiliary axles that are used only as dictated by loading conditions. Consequently, various self-steering axle configurations have been developed, such as are disclosed in U.S. Pat. Nos. 4,770,430 to Lange, and 5,364,113 to Goertzen. These devices utilize positive caster angles for self-steering during forward movement, and shift to negative caster angles during reverse or backing movements. In related developments, reverse-caster mechanisms have been applied to towed vehicles (trailers), as disclosed for example in U.S. Pat. Nos. 3,447,813 to Wolf, 5,474,320 to Bojarski et al., and 6,036,217 to Burkart, Sr., et al. A first embodiment disclosed in the Wolf reference uses a mechanism to distort oppositely disposed load-carrying leaf springs between respective positive and negative camber orientations of an axle beam that is clamped to the springs at medial locations thereof. A disadvantage of this arrangement is the extra and potentially damaging strain to which the springs are subjected. Another disadvantage is that excessive force is required to effect the shifting between camber angles in that the stiffness of the springs must be overcome by the mechanism.
In other embodiments of the Wolf device (and as disclosed in the other references), a separate pivoting element is added to the suspension itself to permit shifting between positive and negative caster. These devices of the prior art are also not entirely satisfactory, typically exhibiting one or more of the following disadvantages:
1. They incorporate excessive amounts of “unsprung weight” that compromises performance on rough roads;
2. They are expensive to provide in that the separate pivoting elements have to be fitted into tight spaces and require elaborate and precise machining operations;
3. They are incompatible in many cases with standard suspension designs and existing parts inventories; and
4. They contribute to vehicle instability in that they add objectionable height to the suspension arrangement.
Further, the Lange reference exhibits a disadvantage of the above-referenced first embodiment of the Wolf device, in that it incorporates air-springs that must be deflected laterally and angularly during camber shifts. This restricts the effective travel range of the suspension and/or contributes to premature failure of the air springs. Moreover, another embodiment of the Wolf device requires the ride height of the suspension itself to be changed in order to effect camber changes. This is particularly impractical in commercial freight-hauling vehicles, both in regard to the energy expended in raising and lowering loads, and the added difficulties associated with operations at loading docks.
Thus there is a need for a self-steering axle unit that operates without excessive or abnormal spring deflection, that is light in unsprung weight, that is versatile in being compatible with conventional suspension configurations, and that otherwise overcomes the disadvantages of the prior art.